Flow safety valves are used in the natural gas industry to prevent explosive pipeline gases such as natural gas, propane, methane, coal gas, town gas, etc. from escaping when a pipe is ruptured. These safety valves will remain open during normal use, when there is back pressure downstream from the valve, but will trip (snap shut) when the downstream pressure disappears. This prevents fires and explosions when gas lines are ruptured. However, some of these safety valves are not designed to shut off the flow completely when tripped. Instead, a small controlled leakage flow is allowed through such a valve to re-pressurized the line after the break is repaired.
The basic idea of small controlled leakage is presented in U.S. Pat. No. 3,918,481, issued to Doe et al. The Doe patent shows a limiter comprised of a hollow valve body with a movable valve head (or float) 22 inside. At the upstream end, where gas enters the valve, is an insert 12 with a circular edge which acts as a seat for the float 22; flutes 28 create gaps between the float and the insert 12, allowing the float to remain seated during normal gas flows. A coil spring 20 pushes the float against the insert.
When the flow is greater the float is pushed downstream and the force of the coil spring is overcome. At the downstream end of the float is a long stem 24, integral with the float, which is inserted into the smaller open end of a conical insert 14. The stem diameter varies with the distance from the valve body; as the float is pushed downstream by the flow of gas, the stem's cross-sectional area variation affects the flow of gas through an annular space 38 between the stem and an insert 14, but does not introduce appreciable pressure drop. (See the Doe '481 patent at col. 1, line 45; col. 2, lines 40-62; and claim 2.)
Doe does not disclose or suggest the use of a cylindrical stem or one of constant or substantially constant cross-section. Neither does Doe disclose or suggest a substantially cylindrical bore in which a stem might be disposed. The stem of Doe does not aid in assembly; instead, it makes assembly difficult.
U.S. Pat. No. 3,438,389 to E. R. Lupin shows a liquid flow metering device with an elongate cylindrical stem 16 disposed within a cylindrical bore. The stem 16 is not movable. An un-numbered base element of greater diameter supports the stem 16. The base element appears to be concentric with the interior of the movable piston 24 for a short distance. This device is not a valve.
U.S. Pat. No. 2,699,799 to Wager shows a scavenging-air valve for a soot blower having a major cylindrical bore 24 and a smaller portion (bore) 29. A part 28 (FIG. 4) slides within the portion 29 with a clearance of 0.010 inch for alignment (col. 2, lines 50-53). In the bore 24 slides a disc-like collar 32 with a free sliding fit (col. 2, line 57). The collar 32 includes channels 35 which leak steam past the collar 32 (FIG. 5); leakage past elements 28 or 32 is not disclosed. This is not a gas line safety valve.
U.S. Pat. No. to Hagan et al., 4,958,657, dated Sep. 25, 1990 and assigned to UMAC, Inc., shows a gas-line flow restrictor comprising a cylindrical pipe or bore within which a valve head member (float) 24 is free to move. The float 24 has a tapered nose 26 which closes against a circular edge to shut down the gas flow when the valve is tripped; a coil spring holds the float away when the flow is less than the trip value. To limit the upstream travel of the float, an annular step 32 acts as a stop for guiding fins 30 of the sliding valve head 24. The cruciform fins 30 guide the float along the bore and allow gas to pass by. The upstream side of the float is not tapered or streamlined. The body or shell has a uniform outside diameter for placement inside a pipe (col. 3, line 35). The Hagan device differs from that of Doe in having the conical nose pointing in opposite directions: the Hagan devices point downstream, whereas the nose of the Doe device points upstream.
Form No. XFS-1090 published by Dresser Manufacturing Division of Dresser Industries, Inc., Bradford, Pa. 16701, shows a style 488-XFS Excess Flow Device (EFD) that is incorporated into a connector as the insert stiffener. The Dresser 488-XFS device consists of a steel body tube which has an annular step for retaining one end of a coil spring. A float or popper has a downstream conical surface that bears against a valve seat formed into the steel body upstream of the step. A retaining washer is crimped or staked into place at the stiffener end against the upstream end of the float or popper. Another model eliminates the washer, in which case the end of the poppet is retained solely by crimping the end of the casing or stiffener.
Assembly of the device requires that the washer, spring, and popper be installed correctly and held in position during the operation of joining the stiffener to the body of the pipe connector. The 488-XFS device is designed to work with Dresser Style 501 compression ends, which include a flat end with a flange set back from the end. Thus, the crimping or staking is minimal and the Dresser devices have the disadvantage that the parts are not sufficiently securely held within the stiffener.
If the Dresser device needs to be dis-assembled in the field (for example, to remove dirt that has fallen into the device) the user will have great difficulties. First, to undo the crimps will be a matter of substantial difficulty. If the crimps are undone and then remade, the metal of the edge may become fatigued and the device may fail. To re-crimp while holding the washer in place would be extremely difficult if at all possible in the field. The prior art also does not disclose a gas line safety valve built into a stiffener that is easy to assemble, securely held, and easily disassembled.
Another problem which has not been addressed by the prior art is that of sharp, but temporary, drops in the downstream pressure caused by snap-acting loads, i.e. not caused by line ruptures. Such fluctuations may cause the prior-art gas line safety valves to trip, shutting off the gas supply. Once the safety valve is tripped, very little gas can move into the low-pressure region; it will then take appreciable time for the limiter to reset. Meanwhile, the appliances downstream are starved of gas.
The prior art does not disclose a gas flow safety valve for natural gas lines and the like which incorporates a structure for increasing the resistance to these fluctuations.